Complications Accompanying Shunts
Chat Highlights
February 1, 2006

On Wednesday, February 1, 2006, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Complications Accompanying Shunts."

Moderator:  Welcome back, Dr. Wilson.  Will you please start the discussion about shunts by describing their function? 

Dr. Rick Wilson:  A shunt is a device that takes fluid from one place and delivers it to another. They were pioneered with cerebral spinal fluid for hydrocephalus.  In the eye, a shunt takes fluid from the anterior or posterior chambers of the eye and passes it through a tube to a plate sewn on the equator of the eye, that is, half way back, protected by the orbital bones.

Moderator:  Are shunts all the same size? 

Dr. Rick Wilson:  Shunts range from infant shunts, which I think are only about 65 mm squared (but don't quote me on that figure) to moderate, at 250 mm squared, to large, at 350 mm squared. We used to have one that measured 500 mm squared, but it didn't give better success and had more complications. The infant shunt is the size of a shirt button. The largest one is as big as your thumb from the tip to the first knuckle.

P:  What kind of material are shunts made of?  Is there a chance the body will reject a shunt as a foreign object?  

Dr. Rick Wilson:  The materials are usually high-grade silicone or polymethylmethcrylate that are inert biologically.  I don't think I have ever seen a real rejection.

P:  Are shunts generally considered only after trabs have been unsuccessful, or are they sometimes done before a trab is tried?

Dr. Rick Wilson:  The indications for a shunt, rather than a trabeculectomy, are as follows: 

1. A failed, well-done, previous trabeculectomy with mitomycin-C 
2. Conjunctiva too scarred to elevate a conjunctival flap
3. Neovascularization in neovascular glaucoma not yet quiescent post PRP (laser photocoagulation), but IOP (intraocular pressure) forces surgery 
4. Recurrent episodes of serious intraocular inflammation
5. Aggressive ICE (irido-corneal epithelial) syndrome
6. Inner ostium of shunt placed on front of anterior chamber IOL (intraocular lens) to avoid vitreous incarceration in patients in whom you do not want to do a PPV (pars plana vitrectomy)
7. Contact lens wear is essential for the patient.

P:  Are shunts being used in young children?

Dr. Rick Wilson:  We are starting to use shunts in young children who don't need really low pressures to control their glaucoma,  because of the risk of long-term bleb infections in someone who might not heed the warning signs.

P:  How is vision affected by shunt surgery, both short term and long term? 

Dr. Rick Wilson:  Usually, vision is not blurred by the insertion of the shunt but by the change in IOP.  If the pressure drops too low, many people develop fluid between the layers of the eye, called a choroidal detachment.  That keeps the IOP abnormally low, and distorts the retina so that vision can be reduced to counting fingers. 

Moderator:  Are most complications with shunt surgeries related to the actual mechanical device or from the recipient's eye not responding as expected?

Dr. Rick Wilson:  The plate keeps the scar tissue from reaching the tube, closing it down.  Since scar tissue develops all around the plate, when aqueous runs out of the tube, it is contained in this pocket of scar tissue.  The thickness of the scar tissue and the size of the plate determine the resultant IOP. 

Moderator:  What are some of the kinds of complications that can occur after shunt surgery? 

Dr. Rick Wilson:  One thing about shunts is that they are just plumbing.  If the shunt isn't working, the inside opening of the tube must be clogged with iris, blood, etc.  You can see the inside opening of the tube to determine if vitreous or something is blocking it.  If the other end of the tube is the problem, usually it is caused by too much scar tissue or compressed scar tissue around the plate that is causing the higher IOP.

P:  How do you clear the tube? 

Dr. Rick Wilson:  If the problem is vitreous, the clear jelly that fills up the back of the eye, sometimes it can be lasered.  If it is blood, or iris, surgery may be needed to remove it.  While the success rate with shunts is higher than it is with trabeculectomy, the IOP is usually not as low as with trabeculectomies.  One or more surgeries may be required to achieve the maximum success rate.

Moderator:  What can be done about the build-up of scar tissue in the area of the plate? 

Dr. Rick Wilson:  We have tried soaking the area with mitomycin, but our studies show that after a few months it helps little.  The British, however, still believe in that technique.

P:  After how many shunts that have failed due to scar tissue do you reconsider whether it is advisable to implant another?  If another shunt is not advisable, what can be tried next? 

Dr. Rick Wilson:  Usually, I use two plates above in a patient with binocular vision, that is, seeing out of both eyes together.  In patients with one eye, I use two plates above and one below, for a total of three.  If the maximum number of plates has not controlled the IOP (intraocular pressure), then we have to add cyclophotocoagulation to reduce the amount of fluid the eye makes to equal the amount of fluid exiting into the shunt reservoirs. 

P:  What is cyclophotocoagulation? 

Dr. Rick Wilson:  Cyclophotocoagulation is a procedure in which a laser is applied to the ciliary body (the part of the eye that makes fluid) so the eye does not make as much fluid.

P:  Three shunts seem to me to be a high number when two shunts have already failed due to the formation of scar tissue.  What is the thinking when deciding on a third?  Is cyclophotocoagulation or ECP (endoscopic cyclophotocoagulation) that much more risky?  Why not do that after one failed shunt?

Dr. Rick Wilson:  We usually add plates if the first plate was partially successful; for example, the IOP (intraocular pressure) decreased from 44 to 28 mm Hg. Perhaps increasing the area from drainage would control the IOP.  It is important to wait past the "high bleb phase," when the scar tissue around the plate is dense.  After four months, the scar tissue may remodel, with more pores for the aqueous to leak through, and the IOP may drop without any manipulation.

P:  Can a bleed during shunt surgery be prevented if done more gently?  My mother said it felt like the shunt was jammed in.  Unfortunately, now, 2 1/2 years and five surgeries later, she has no sight in the only good eye she had.  She's very depressed. 

Dr. Rick Wilson:  Usually, bleeding is an issue only in those patients with neovascular glaucoma who have had a vein occlusion in the retina, or bad diabetes that causes new and fragile vessels to grow on the iris surface and over the trabecular meshwork, or in older patients if the IOP drops too low after the shunt starts working. Without the support of the pressure in the eye pushing the middle layer of the eye, the choroid (which is made up entirely of vessels) against the wall of the eye, a weak spot in the vessels can rupture and cause bleeding between the sclera and the retina.  That can either be devastating, or only a short-term problem, depending upon the extent.

P:  What is the range of the post-operative pressure that may cause the choroidal bleeding?

Dr. Rick Wilson:  It varies with the health of the choroid.  Most people have bleeding with IOPs under 8 mm Hg.  The lower the IOP, the less support for the choroid, and the more dangerous the situation.

P:  If there is no diabetes and no history of being a bleeder, why  would bleeding occur? 

Dr. Rick Wilson:  Cholesterol lining the vessel wall may make the vessels stiff.  When the IOP is low, eye movements cause the eye wall to flex, bending those vessels that are not used to being bent and possibly causing them to break.  Bleeds often occur when the patient is sleeping and dreaming, that is, REM (rapid eye movement) sleep.

Moderator:  Thank you for coming tonight even though you have the flu.  We wish you a speedy recovery.  

On February 8, Dr. Wilson discussed "Complications Accompanying Lasers" in the Chat room. Click here for highlights of that meeting.

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